can you download aircraft in the Qt launcher? If so then the appropriate catalog has been found and the in-sim aircraft center should just work. But the main concern should be the Qt launcher as this is the future direction of this feature as I understand it.

Let's just hope that the removal of the Canvas-based Aircraft Center was coordinated with TheTom, especially given that the Canvas Aircraft Center -unlike its Qt5 replacement, wasn't exactly known for "crashing" FlightGear ...